吸(xī) - to inhale, to suck, to absorb in Mandarin

吸 (xī) is the drawing-in verb. Air into lungs, liquid up a straw, water into a cloth: anything pulled inwards by suction. It is the in half of 呼吸, where 呼 is the breath going out.

On its own it usually needs an object or a complement: 吸气, 吸水, 吸一口. It also heads a family of compounds worth knowing early, especially 吸引 for attracting attention and 吸收 for absorbing nutrients or information.

The trap is smoking. 吸烟 is correct but formal, the wording you see on a no-smoking sign; in speech people say 抽烟. And do not stretch 吸 to cover a deep breath as an English speaker might use inhale for the whole action, because the noun phrase 一口气 is doing most of that work.